The dimensions of electronic circuit elements have decreased rapidly over the past half century. Familiar circuit elements, including resistors, capacitors, inductors, diodes, and transistors that were once macroscale devices soldered by hand into macroscale circuits are now fabricated at sub-microscale dimensions within integrated circuits. Photolithography-based semiconductor manufacturing techniques can produce integrated circuits with tens of millions of circuit elements per square centimeter. The steady decrease in size of circuit elements and increase in the component densities of integrated circuits have enabled a rapid increase in clock speeds as which integrated circuits can be operated as well as enormous increases in the functionalities, computational bandwidths, data-storage capacities, and efficiency of operation of integrated circuits and integrated-circuit-based electronic devices.
Unfortunately, physical constraints with respect to further increases in the densities of components within integrated-circuits manufactured using photolithography methods are being approached. Ultimately, photolithography methods are constrained by the wave length of radiation passing through photolithography masks in order to fix and etch photoresist and, as dimensions of circuit lines and components decrease further into nanoscale dimensions, current leakage through tunneling and power-losses due to relatively high resistances of nanoscale components are providing challenges with respect to further decreasing component sizes and increasing component densities by traditional integrated-circuit-manufacturing and design methodologies. These challenges have spawned entirely new approaches to the design and manufacture of nanoscale circuitry and circuit elements. Research and development efforts are currently being expended to create extremely dense, nanoscale electronic circuitry through self-assembly of nanoscale components, nanoscale imprinting, and other relatively new methods. In addition, new types of circuit elements that operate at nanoscale dimensions have been discovered, including memristive switching materials that can be employed as bistable nanoscale memory elements. Initially, memristor implementations were constrained by relatively low switching frequencies, but newer techniques for manufacturing memristive memory, elements and other memristor-based devices have produced memristive devices that can be switched at gigahertz frequencies. Efforts continue to improve both switching times and power efficiency of memristive memory elements and other nanoscale electronic devices that incorporate dielectric materials with multiple, stable electronic states.